WASHINGTON - Mitt Romney carries many advantages into an Illinois primary on Tuesday that is shaping up to be a vital test of the next phase of the volatile Republican nominating contest. The state is chock full of the wealthy, suburban, and educated voters who have favored him so far. He has secured top endorsements, he has a lead in the polls, and he brings an economic message to a state struggling economically.

But with increasing worry that Rick Santorum could score another upset, the former Massachusetts governor has rejiggered his schedule to spend much of the next four days campaigning in Illinois. Romney and his allies have significantly increased their advertising, outspending Santorum 7 to 1, and are ramping up the type of negative attacks that have been a hallmark of Romney’s other narrow wins.

“We cannot afford a Republican version of President Obama. And Rick Santorum is a Republican Obama,’’ John Sununu, former governor of New Hampshire, who has been the most aggressive surrogate for the Romney campaign, told reporters Friday on a conference call. “We hope that the voters in Illinois take a good, hard look at their responsibility.’’

Although there are caucuses Saturday in Missouri and Sunday in Puerto Rico, it is the Illinois primary on Tuesday that is the next big prize on the calendar. It marks yet another opportunity for Santorum to deny Romney an expected win, escalating doubts over the Bay Stater’s candidacy. It also allows Romney a chance to extend his growing lead in delegates, continue making the case to establishment Republicans that they should rally behind his campaign, and regain some momentum after losing two key Southern states last week.

Illinois provides yet another case where Romney dominates in nearly every traditional political metric - number of ads, endorsements, and boots on the ground - but where Santorum is remaining competitive with a scrappy campaign that taps into the lingering antipathy toward Romney.

Santorum, after leaving Puerto Rico to campaign in Missouri and Illinois, characterized himself on Friday as the more electric leader who could energize the base, even while having fewer resources.

“We’re being outspent 10 to 1 on television in Illinois, and I still think we have a chance to win Illinois,’’ Santorum told a rally in Osage Beach, Mo., according to the Associated Press. “Because without a vision the people will perish.’’

Romney has repeatedly characterized Santorum as a politician who has little insight into how to create a robust economy.

“I think you’ll find he’s an economic lightweight,’’ Romney said this week on a telephone town hall with Illinois voters. “He really doesn’t understand fundamentally what it takes to make this economy grow, and thrive, and add jobs.’’

“The economy,’’ Romney added, “is in my wheelhouse.’’

In one of the few polls taken after Santorum’s wins in Alabama and Mississippi on Tuesday, Romney was leading, but Santorum was still within striking distance.

Romney has support of 37 percent of likely primary voters, compared with 31 percent for Santorum, according to a Fox Chicago News poll released on Thursday. Newt Gingrich had 14 percent, and Ron Paul had 8 percent; 10 percent of voters were undecided.

The poll, conducted by Springfield, Ill.-based pollster We Ask America, had a 2.2 percentage point margin of error. Nearly half of voters said they were unsatisfied with their choices and wished some other candidate were running instead. Real Clear Politics’ average of recent polls has Romney at 37.7 percent and Santorum at 31.3 percent.

Illinois leans Democratic and is unlikely to be competitive in the general election, but Obama on Friday returned home to Chicago for a fund-raising event. He tweaked Republicans for not hewing more closely to their party’s forefather, Abraham Lincoln.

“My message to all the candidates is, ‘Welcome to the Land of Lincoln.’ Because I’m thinking maybe some Lincoln will rub off on them while they are here,’’ Obama told a crowd at the Palmer House Hilton. “You may be watching some of this avalanche of attack ads and think this is not appealing to the better angels of our nature, but hope springs eternal.’’

The results this Tuesday will also further define which candidate is able to attract voters in Midwestern states filled with the independents and working-class voters who can drive general elections. Romney narrowly won Ohio and his native Michigan, but Santorum squeaked out a win in Iowa and won handily in Minnesota.

The battle in Illinois will largely hinge on whether Romney can drive up numbers in the Chicago area, where the majority of the state’s vote typically comes from, or whether Santorum can attract voters in the more conservative southern counties.

There are 54 delegates that will be directly allotted on Tuesday on a proportional basis, with another 15 to be named at the state party convention in June.

Romney supporters are hoping that the state’s high unemployment rate - it was 9.4 percent in January, above the national average - and falling housing prices will make the electorate more receptive to a former businessman.

Romney had earlier planned on spending much of the weekend in Puerto Rico, but shifted his schedule to add several events in Illinois. He campaigned in the state on Friday morning before flying to Puerto Rico, and he will return on Saturday for more campaign events.

Romney and his political allies are also vastly outspending Santorum and his. Romney and the independent political action committee supporting him have spent $3.7 million in the state, according to a source monitoring ad buying. Santorum and the PAC supporting him, by contrast, have spent about $515,000.

Romney’s ads are hard-hitting and direct. One radio ad mocks Santorum for losing his 2006 Senate reelection bid by 18 points. Romney’s campaign on Friday released a new ad that directly challenges Santorum over his economic know-how.

“Santorum’s real weakness is the economy. He’s never run a business or a state,’’ a deep-voiced narrator says, as eerie piano music plays and images of closed factories and a young child peering out of a crib flash across the screen. “His plan? Economic illiteracy.’’

“Rick Santorum, another economic lightweight,’’ the ad says.

Meanwhile, the super PAC supporting Santorum has a biting ad that criticizes Romney for his health care plan in Massachusetts, for raising fees as governor, and for supporting the Wall Street bailout.

“Mitt Romney,’’ the narrator says, as close-up, grainy photos of Romney’s temples are shown. “More debt and taxes. Less jobs. More of the same.’’

Romney has significantly increased his media appearances, targeting local markets and appearing so often on Fox News that NBC’s David Gregory on Friday said Romney was becoming “a Fox News contributor, apparently.’’ Over the past two days, Romney has had at least four television interviews and a radio interview with Fox News, and on Sunday he’ll be on “Fox News Sunday.’’

Romney is the only candidate this campaign who has yet to agree to an interview on “Meet the Press,’’ the show Gregory hosts and one where probing questions make it a political rite of passage for most candidates.